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Some supplements contain too much vitamin C

It's cold and flu season, which means many people are reaching for vitamin C, as it may modestly reduce cold symptoms (although it won't prevent a cold). What they may not know is that taking too much vitamin C may also cause stomach upset and diarrhea and, if used long-term, potentially, other negative effects.

To make sure that supplements contain and deliver the amounts of vitamin C listed on their labels, ConsumerLab.com recently tested a variety of vitamin C supplements. Many top-quality products were identified, but two vitamin C supplements were discovered to contain 40 percent more vitamin C than listed, increasing the chance that they could cause side effects in some people. ConsumerLab.com also discovered that one supplement contained 15 percent less vitamin C than listed.

Most adults need only 75 mg to 120 mg of vitamin C per day, which is roughly the amount in a glass of orange juice. Children need just 15 mg to 75 mg. A much higher dose (1,000 mg or more) taken daily throughout the cold season has been shown to reduce the symptoms and duration of a cold, which is why many products are designed to deliver amounts ranging from 250 mg to 1,000 mg per unit. However, side effects may occur when daily intake (including that from foods as well as supplements) exceeds just 400 mg for very young children and 2,000 IU for adults.

ConsumerLab.com also found that a common dose of vitamin C from a high-quality supplement can cost as little as one cent, or over one dollar.

ConsumerLab.com's Vitamin C Supplements Review is available online. It includes quality ratings and comparisons of 25 vitamin C supplements and explains: